Virtually all modern bicycles transmit power from the rider's legs to the rear wheel by means of a chain connecting a gear attached to the rider's feet by means of pedals and crankarms to a second gear connected to the rear wheel. Bicycles having more than three possible gear ratios generally include a cluster of different gears at the rear wheel. The rider can choose which gear of the gear cluster the chain will engage by means of a shift lever or other control device. That selection device is attached to a derailleur, which guides the chain to the appropriate gear.
The derailleur generally consists of a pair of small pulleys or jockey wheels located underneath the gear cluster and a means for moving the jockey wheels across the sequence of planes defined by the gear cluster. Because the appropriate length of chain will change depending upon the gear selected, the cage which holds the jockey wheels is attached to the derailleur body by means of a spring, which allows the jockey wheels to hold the chain in relatively constant tension and thereby prevent it from tangling or falling off the gear sprockets.
In traditional derailleur systems, the derailleur is generally attached to the selector mechanism by means of a cable. The cable is generally held in tension by a spring located in the derailleur body and which tends to move the jockey wheel assembly or mechanism towards the outside of the gear cluster. The shifting mechanism uses high internal friction to counter the spring tension in the derailleur and allow the derailleur to remain in the position set by the rider. Such a system is infinitely variable, and requires the rider to acquire considerable skill in order to change gears quickly and reliably.
In the 1980's, Shimano introduced an "indexed" shifting system. This system used a series of detents in the gear shift lever to indicate and hold the precise location of the various rear gears. An audible "click" allowed the rider to easily sense when a gear was selected.
The Shimano-type indexing system requires that the jockey wheels in the rear derailleur be precisely positioned under the appropriate sprocket for each gear. In order to reduce the degree of precision required, Shimano equipped its derailleurs with an upper jockey wheel that was permitted to float laterally by approximately 0.6 millimeter. This float allowed the system to operate despite slight misalignment of the derailleur body due to improper cable tension or other improper adjustment. The jockey wheels used by Shimano all rotate around plain bushings. These bushings generate considerably more friction than bearings with rolling elements, and tend to squeak and make other objectionable noises, particularly if not regularly cleaned and lubricated.
Since their introduction by Shimano, a number of other component manufacturers, including Campagnolo, have introduced indexed shifting systems. All employ a floating jockey wheel using plain bushings.
A number of manufacturers such as Bullseye, Control Tech and Carmichael, have marketed derailleur wheels that include sealed ball bearings. These units generate much less friction than do the stock wheels using plain bearings. However, they do not allow the float that is essential to the proper functioning of the indexed shifting systems. Simply leaving the inner race of the bearing loose on its mounting bolt would be ineffective because of the phenomenon called "stiction" or static friction. The seals that are required in order to protect the rolling elements of the bearing from contamination cause little friction once the bearing has begun to turn, but have high initial friction when they begin to turn after being stationary. If the inner race of the bearing is sufficiently loose on its mounting bolt to allow lateral movement, the stiction in the seals will tend to make the inner race of the bearing rotate relative to the bolt. This makes the assembly function as a plain bearing, and negates the advantages of the rotating elements.
Ideally, a jockey wheel would use a bearing containing rolling elements to reduce friction and allow lateral float for precise shifting. The construction of the jockey wheel would be such as to induce the outer race to rotate relative to the inner race, thereby to ensure that it functions as a roller or ball bearing.
These and other objects of the present invention will be apparent to those skilled in the field from the following detailed description of a preferred embodiment.